1. Field of the Invention
The present invention relates to a method for producing a III-IV group compound semiconductor layer, a method for producing a semiconductor light emitting element and a vapor phase growing apparatus.
2. Description of the Related Art
Conventionally, as a method for growing III-IV group compound semiconductor crystals having less crystal defects on a GaAs substrate, Japanese Laid-Open Publication No. 8-203837 describes a following method.
FIG. 9 is a diagram showing a pattern in substrate temperature and material gas introduction during the growth over time in a conventional method for producing a III-IV group compound semiconductor layer disclosed in Japanese Laid-Open Publication No. 8-203837.
First, a GaAs substrate is placed in a reaction container (reaction chamber). The atmosphere in the reaction container is reduced to desirable pressure. Then, as the V group material gas, AsH3 (arsine) is introduced into the reaction container. Next, substrate temperature of the GaAs substrate is increased to 650° C. TMG(trimethyl gallium) is introduced to the reaction container as III group material gas to grow a GaAs buffer layer on the GaAs substrate. Then, supply of TMG to the reaction container is stopped and the growth of the GaAs buffer layer is stopped. At the same time as the supply of the AsH3 is stopped, introduction of PH3 (phosphine) as a V group material is started. After a predetermined time period, TMG, TMA (trimethyl aluminum), and TMI (trimethyl indium) are supplied to the reaction container as III group material gases to start the growth of a AlGaInP layer. During the growth process, the substrate temperature of the GaAs substrate is increased to 750° C. With the substrate temperature of the GaAs substrate maintained at 750° C., the AlGaInP layer is grown until it has a predetermined thickness.
In the conventional method for producing a III-IV group compound semiconductor layer disclosed in Japanese Laid-Open Publication No. 8-20387, switching from AsH3 to PH3 is performed at a substrate temperature lower than a preferable growth temperature for the AlGaInP. The reason is as described below.
When AsH3 is absent at high substrate temperature, As tends to dissociate from GaAs crystal. Thus, if switching from AsH3 to PH3 is performed at high substrate temperature, As atoms dissociate immediately after the switching to PH3. These may cause crystal defects. If the supply of TMG, TMA, and TMI is started in a short time after the switching to PH3 in order to suppress the dissociation of As atoms so as to grow AlGaInP, gases cannot be fully exchanged from AsH3 to PH3 in the reaction container. Thus, As atoms are mixed in AlGaInP. These may cause crystal defects.
Therefore, in the conventional technique disclosed in Japanese Laid-Open Publication No. 8-20387, the gas is switched from AsH3 to PH3 at low substrate temperature. Thus, As leakage immediately after the switching from AsH3 to PH3 or As atom insertion into AlGaInP can be suppressed compared to the case of switching at the preferable growth temperature for the AlGaInP. As a result, it becomes possible to obtain crystals having good quality.
As another example of a method for growing III-IV group compound semiconductor crystals having less crystal defects on a GaAs substrate, Japanese Laid-Open Publication No. 2000-216496 describes a following method.
FIG. 10 is a diagram showing a pattern in substrate temperature and material gas introduction during the growth over time in a conventional method for producing a III-IV group compound semiconductor layer disclosed in Japanese Laid-Open Publication No. 2000-216496.
First, a n-GaAs substrate is placed in a reaction container. The atmosphere in the reaction container is reduced to desirable pressure. Then, as the V group material gas, AsH3 is introduced into the reaction container.
Next, the first GaAs substrate is heated until the substrate temperature is increased to 770–830° C., a first temperature at which n-GaAs buffer layer is to be grown.
Then, with the substrate temperature maintained at the first temperature, TMG is introduced to the reaction container as III group material gas to grow a first n-GaAs buffer layer on the n-GaAs substrate until it has a predetermined thickness. Then, supply of TMG to the reaction container is stopped and the growth of the first n-GaAs buffer layer is stopped.
Further, the substrate temperature is reduced to a second temperature, 680° C. TMG is supplied into the reaction container to grow a second n-GaAs buffer layer.
Then, V group material gas is switched from AsH3 to PH3. After 10 seconds, TMG, TMA, and TMI are supplied to the reaction container as III group material gases to start the growth of a AlGaInP layer. During the growth process, the substrate temperature of the GaAs substrate is increased to 760° C. With the substrate temperature of the GaAs substrate maintained at 760° C., the AlGaInP layer is grown until it has a predetermined thickness.
In the conventional method for producing a III-IV group compound semiconductor layer disclosed in Japanese Laid-Open Publication No. 2000-216496, the first n-GaAs buffer layer is grown at a substrate temperature higher than the preferable growth temperature for the AlGaInP. The reason is as described below.
By growing the first n-GaAs buffer layer at a higher substrate temperature prior to the growth of AlGaInP, oxygen attached to substrate holding member(s) and the like can be evaporated during the growth of the first n-GaAs buffer layer. Since the second n-GaAs buffer layer and the AlGaInP layer are grown at a substrate temperature lower than the first n-GaAs buffer layer, no oxygen evaporates from the substrate holding member(s) and the like.
Therefore, according to the conventional technique disclosed in Japanese Laid-Open Publication No. 2000-216496, it is possible to obtain AlGaInP crystals forming a nonluminous recombination center as AlGaInP crystals having good quality with less oxygen mixed therein.
Further, Japanese Laid-Open Publication No. 6-244122 discloses a method for growing a GaAs buffer layer on an Si substrate. In the method, GaAs layer is grown even during a temperature increasing step, in which the substrate temperature is increased from a temperature at which the material gas can be decomposed to the growth temperature at which the single crystal can be grown, by continuously supplying a material gas during the temperature increasing step. According to the conventional technique disclosed in Japanese Laid-Open Publication No. 6-244122, a GaAs layer having good surface flatness can be grown on an Si substrate.
In the conventional technique disclosed in above-mentioned Japanese Laid-Open Publication No. 8-20387, crystals having good quality are obtained by preventing As from being mixed into AlGaInP or dissociated at an interface of GaAs and AlGaInP. In the conventional technique disclosed in above-mentioned Japanese Laid-Open Publication No. 2000-216496, crystals having good quality are obtained by preventing oxygen from being mixed into AlGaInP at an interface of GaAs and AlGaInP.
However, through further research, the present inventors found that a phenomena which is not considered in the above conventional techniques is causing deterioration in crystallinity of AlGaInP.
FIG. 11A is a cross-sectional view showing a structure of important parts of a vapor phase growing apparatus 1100 used in growing a compound semiconductor. FIG. 11B is a plan view showing the same.
The vapor phase growing apparatus 1100 includes a reaction chamber (reaction container or reactor) 501 and a substrate holding member (substrate receptacle) 502 provided in the reaction chamber 501. The substrate holding member 502 holds a GaAs substrate. The material gas is supplied from a gas inlet port 501a provided in the reaction chamber 501 and exhausted from a gas outlet port 501b provided in the reaction chamber 501 and is made to flow from one side to the other side with respect to the GaAs substrate. When the gas is supplied to the reaction chamber 501 to grow crystals, reaction products attach not only to the substrate but also to other portions in the reaction chamber 501, such as the substrate holding member 502, a surface of the reaction chamber 501 opposing the substrate, and the like.
The reaction product attached to the reaction chamber 501 may decompose or dissociate and re-evaporate depending on the conditions. The re-evaporated gas may be mixed into the material gas. The re-evaporated gas generated on the downstream side (the side of the gas outlet port 501b) of the substrate is evacuated directly from the reaction chamber 501. The re-evaporated gas generated on the upstream side (the side of the gas inlet port 501a) of the substrate is mixed into the material gas and reaches a surface of the substrate.
The influence of the re-evaporated gas is most strong when the re-evaporated gas reaches the substrate surface on which the semiconductor layer is not growing, such as before the growth or during the interruption of the growth of the semiconductor layer. When the semiconductor layer is growing, elements generated by re-evaporation (hereinafter, referred to as re-evaporated elements) are uniformly mixed into the growing layer. Thus, the concentration of the re-evaporated elements is relatively low. When the semiconductor layer is not yet grown or growth is interrupted, the re-evaporated elements attach to the substrate surface or a crystal surface and are accumulated until the next growth step. Thus, the re-evaporated elements are accumulated on the growth interface at a high concentration.
For example, in the case where In is re-evaporated during the temperature-increasing step before the start of the growth and reaches the GaAs substrate, In reacts and is grown with AsH3 which is supplied during the temperature-increasing step and forms InAs on the surface of the GaAs substrate. Since InAs has a large lattice mismatch with GaAs,crystals are not grown normally. Thus, crystal defects are generated. A GaAs buffer layer and a AlGaInP layer are grown on such crystal defects. Thus, crystals having good quality cannot be obtained.
In the case where the growth of, for example, AlGaInP is interrupted due to a change in composition or the like, supply of the III group material component is stopped to stop the growth with PH3 being supplied. When As is supplied from upstream while PH3 is supplied, As attaches to a surface of the AlGaInP, or a part of phosphorous in the AlGaInP is substituted with As. Thus, crystal defects are generated at the growth interface.
The present inventors conducted detailed research and have found that reaction products which may attach to portions other than the substrate in the reaction chamber are generated with any element, but attachment and re-evaporation of the As and In are remarkable compared to other elements and may cause a significant effect on quality of crystals.
The conventional technique disclosed in Japanese Laid-Open Publication No. 6-244122 is a technique for improving a surface flatness of GaAs on a Si substrate by growing the GaAs layer on the Si substrate with the temperature being increased. It is not directed for preventing impurities from being mixed into a III-V group compound semiconductor layer to improve crystallinity.